The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to the collection of marine seismic data in deep water environments.
Marine seismic exploration requires acoustic source generators for delivering energy to subsurface geologic formations and boundaries. The acoustic energy is discharged along shot lines in the desired survey region and is reflected by the subsurface formations and boundaries. The reflected energy propagates upwardly and is detected with hydrophones or bottom cable geophones. Data from adjacent shot lines are processed to construct an overall geologic image of the subsurface geologic formations.
In shallow water up to one hundred meters deep, bottom cable systems use geophones and hydrophones laid on the sea floor with seismic cable cables. Cable crews connect each cable section as the bottom cables are deployed, retrieved and repaired. One or more vessels deploy the geophones and cable in the selected locations and retrieve the geophones and cables after the selected area is surveyed. In deep water exceeding one hundred meters water depth, conventional bottom cable systems are not useful because such systems require expensive, complex connectors which fail due to leakage and cross-feeding of electrical connections. Failure of a single connector requires repair of the entire cable, significantly increasing survey downtime and the resulting survey cost.
In addition to technical difficulties inherent in bottom cable systems, the size, weight and handling requirements of bottom cable systems are difficult to implement in deep water environments. To overcome these limitations, marine seismic vessels are conventionally used in deep water to perform seismic surveys. Marine seismic vessels tow acoustic energy sources such as compressed air guns through the water. The vessels also tow one or more seismic streamer cables along the selected survey line. The streamers typically range between three and eight kilometers long, and tail buoys attached to the free streamer ends incorporate radar reflectors and navigation and acoustic transponders. Each streamer contains multiple hydrophones which receive the reflected energy emitted by the energy source. The hydrophones are typically wired together in receiver groups regularly spaced along the streamer. To account for vessel movement, data recording and processing calculations require time and position correlations for each active component of the seismic data gathering system.
Variations in the lateral spacing between adjacent streamers due to environmental forces and vessel course changes introduce variables in the collected data. The actual spacing between receiver groups is critical for an accurate analysis of geophysical data. Because the acoustic energy reflections propagate through the water in a towed streamer system, noise significantly distorts the reflected energy. The problems associated with undesirable noise is well known. For example, U.S. Pat. No. 4,970,696 to Crews et al. (1990) disclosed a three dimensional seismic survey system having multiple seismic receivers. Undesirable noise was characterized with uniform sampling intervals and the recorded responses were processed to remove the undesirable noise. In other systems, additional processing is required to account for additional noise, and the quality of maps created from the processed data is reduced.
Other systems have been developed to collect data in a marine environment, and to return the data to the water surface. U.S. Pat. No. 4,007,436 to McMahon (1977) disclosed a flexible sheet for holding hydrophones. U.S. Pat. No. 4,692,906 to Neeley disclosed an ocean bottom seisometer having a weighted skirt which also provided a buoyant space for retrieving the seisometer to the water surface. U.S. Pat. No. 5,189,642 to Donoho et al. (1993) disclosed a seafloor seismic recorder having a chassis which lowered geophones into contact with the seafloor. A geophone package was embedded into the seafloor, and a control package and chassis was raised above the seafloor surface with a leg extension to isolate such components from the geophone package. A ballast ring returned the geophone package to the water surface. Additionally, U.S. Pat. No. 5,696,738 to Lazauski (1997) disclosed a sensing device in contact with the seafloor.
Other systems have been developed to operate and to collect seismic data from multiple recorders. U.S. Pat. No. 4,281,403 to Siems (1981) disclosed a plurality of remote seismic recording units activated with local clocks in each local recording unit, together with a master clock in a central station. U.S. Pat. No. 5,623,455 to Norris (1997) disclosed remote units connected to a plurality of receivers for collecting and transmitting seismic data over a selected transmission channel. U.S. Pat. No. 5,724,241 to Wood et al. (1998) disclosed a distributed data acquisition system having a plurality of recorders for detecting and recording seismic data. The data was collected and continuously collected by data acquisition modules.
In addition to the systems described above, bottom cables are used in relatively shallow water depths to communicate between underwater recorders and equipment located on board a marine seismic vessel. However, bottom cables are heavy because such cables contain insulated lines for transmitting power and signals between the marine recorders and the seismic vessel. The weight, size and reliability concerns of bottom cables, together with the operational expense of deploying and retrieving bottom cables, effectively prevents conventional bottom cable systems from being deployed in deep water.
Existing marine seismic systems do not efficiently position multiple recorders in a deep water array. Accordingly, a need exists for an improved deep water seismic data collection method. The method should preserve the quality of data collection so that subsequent geophysical data processing is enhanced.